Many electronic devices are fabricated by forming layers of material over other layers of material, or over electronic components formed over a substrate. For example, a semiconductor light emitting device, such as a light emitting diode or a laser, can be fabricated by forming epitaxial layers of material over a substrate. In another example, an integrated circuit is formed from a number of layers formed on a substrate. The material layers include layers of semiconductor material, dielectric material, contact material, etc.
In one fabrication technique, thin layers of material are applied by dissolving a solid in a solvent, thus forming a solution, and then spinning the solution onto a surface. Typically, after spinning, the solvent is removed by, for example, mechanical inertia or evaporation, leaving a thin layer of the solid material as layer of the device. Other methods for applying material layers also exist.
In many cases the substrate surface on which the solid layer of material is formed is not planar. Typically, the substrate surface has a non-planar surface topography. For example, if the device is an integrated circuit, the substrate surface topography is characterized by the shapes of circuit elements having differing heights, profiles and areas. The surface of a layer of material applied over such a substrate surface of non-planar topography will typically have a topography that conforms, at least to some degree, to the topography of the substrate surface. A comparison of the 2-dimensional spatial Fourier transforms of the substrate surface topography and the applied material layer reveals that the surface topography of the applied layer has a spatial spectrum that is somewhat attenuated at high frequencies compared to the spectral density of the underlying surface, but otherwise generally conforms to the surface topography of the underlying surface.
However, in some applications, such as in semiconductor manufacturing, it is desirable that the surface of an applied material exhibit a substantially planar surface.
Several prior techniques have been used in an attempt to planarize a material layer formed over a non-planar substrate surface. In one method, the physical properties of the solution applied over the non-planar substrate surface, such as viscosity, density, and molecular weight of the solids in the solution, are designed to reduce the high-frequency spectral content of the 2-D Fourier transform of the surface topography of the applied layer. This technique is effective at planarizing the surface of a layer applied over a non-planar substrate surface if the surface topography of the substrate surface has no spatial frequency components below the cut-off frequency of the low-pass filtering effect of the spun-on solution.
Another prior technique for planarizing a substrate surface having a non-planar topography applies multiple layers, optionally solidifying each layer prior to forming the following layer. The multiple layers can be etched to reduce their thickness. Unfortunately, this technique suffers from the same shortcomings as described above.
A third prior technique attempts to planarize the spun-on layer by using gravitational effects and/or the surface tension properties of the solution by minimizing the viscosity of the solution after it has been spun onto the substrate surface. However, this technique also suffers from the above-described shortcomings.
In a fourth prior technique, a surface of an applied layer is planarized using chemical-mechanical polishing. Unfortunately, it is difficult to remove a precise amount of material and this technique often abrades the substrate surface, thereby damaging the substrate surface.